Research Overview

For the past 50 years, Carver Mead has dedicated his research, teaching, and public
presentation to the physics and technology of electron devices. This effort has been
divided among basic physics, practical devices, and seeing the solid state as a medium for
the realization of novel and enormously concurrent computing structures. Listed below
are a number of important contributions that were made over that period.

1960

Proposed and demonstrated the first three-terminal solid-state device operating with
electron tunneling and hot-electron transport as its operating principles (1).

1961

Initiated a systematic investigation of the energy-momentum relation of electrons in
the energy gap of insulators (2, 3, 4) and semiconductors (5, 6, 7). These studies involved
exquisite control of nanometer-scale device dimensions, albeit in only one dimension.

1962

First demonstration that hot electrons in Gold retained their energy for distances in
the nanometer range (8).

1963

With W. Spitzer (9, 10, 11), established the systematic role of interface states in
determining the energy at interfaces of III-V compounds, independent of the detailed
nature of the interface. This work anticipated the role of interfaces in band-gap
engineering, which is centrally important in modern heterojunction devices.

1963-1976

With W. Spitzer and many other collaborators and students, undertook a
systematic study of the physics and commercial importance of Schottky barriers on a
wide range of semiconductors. The understanding gained from these studies ramified in
several directions over the following years. Much of the nano-scale work accomplished
during the period is directly dependent on knowledge of barrier behavior and tunneling.

1965

Built the first working Schottky-barrier-gate field-effect transistor (12). This device
(MESFET) has come to be the standard high-frequency transistor used in satellites, cell
phones, and other microwave communications systems. Using modern band-gap engineered
materials, the device is now known as the HEMT.

1969

Gave the first systematic treatment of ohmic contacts to semiconductor devices.
These structures were the first true nanometer-scale devices, and are still the most
numerous. Showed that they were tunneling junctions and were critically dependent on
the doping density of the semiconductor, but not on the metal used (13, 14).

With S. Kurtin and T.C. McGill, demonstrated several manifestations of a
fundamental transition in the nature of solids depending on the relative contribution of
covalent and ionic bonding. This paper was highly controversial at the time, and was only
published after careful review by, and discussions with John Bardeen (15).

1970

With S.T. Hsu and R. Whittier, reported the first single-electron transistors. These
were accidental three-dimensional nanometer-scale devices that resulted in field returns
of commercial transistors (16).

Taught first VLSI course at Caltech. First Multi-Project Chip. This course and the
multi-project shared-wafer methodology that went with it became the model for an entire
generation of courses that contributed greatly to innovation around the world (17).

With M. Delbruck, initiated a program to apply the physical principles learned from
electron transport through insulating films to ion transport through membranes of
biological interest. At the time the common belief was that the exponential current-voltage
characteristics of these systems was due to the individual properties of certain
nanometer-scale molecules embedded in the membrane. With a number of collaborators
(18, 19, 20) established that the characteristics were due to the population statistics of the
molecules, and that the individual molecules had an ohmic current-voltage curve. This
result is now taken for granted in the biophysics literature, but was quite controversial at
the time.

1971

With B. Hoeneisen, showed that Silicon Carbide Schottky-barrier diodes with
nanometer-scale depletion layers were vastly more effective high-power rectifiers than
conventional Silicon devices (21). This prediction was based on a deep understanding of
the tunneling process and measurements of barrier properties on this remarkable material.
The potential of these devices was not realized until twenty years later when single-crystal
SiC wafers became available. Today these devices are the workhorses of highpower
electronics.

With S. Colley, built and demonstrated the first simple Silicon Compiler. Produced
both simulation and layout from higher-level functional description (in this case, finite state
machine code). Silicon Compilation was to be the centerpiece of the next 15 years’
work, motivating many of the more detailed contributions. By 1991, every major chip design
effort in the world used some variant of this key technology.

1972

As the culmination of many years of work in solid-state device physics, published
(with B. Hoeneisen) the first prediction of the nanometer-scale lower limit to the size of
transistors (22). These limits were based on fundamental physical laws, and were much
smaller than generally expected. These predictions, along with the general notions of
scalability that went with them, were instrumental in setting the industry on its path
toward nanometer-scale technology. The limits established at that time have held up to
the present day, in the face of many years of experimental and theoretical work done at
laboratories throughout the world (23). Because these results formed the scientific basis
underlying Moore’s Law, they have had enormous economic impact worldwide.

With A. Mohsen, T. McGill, and Y. Daimon, gave the first quantitative treatment of
charge transfer efficiency in overlapping-gate CCD structures (24, 25, 26). A new
clocking methodology was invented, which allowed a considerable increase in both
charge capacity and transfer efficiency (27). This work formed the basis for the high
transfer-efficiency CCD devices now used for imaging applications.

1976

Described two unique concurrent computing structures: a serial compare-under mask
chip (class project from the first VLSI course, actually finished in 1971), and (with
E. Cheng and R. Lyon), a serial pipelined multiplier. Articulated the concept that
pipelined structures passing data to nearest neighbors formed an optimal VLSI structure.
This principle paved the way for much more general work on systolic algorithms.

1977

Distilled many of these thoughts into an article, written jointly with Ivan
Sutherland, in Scientific American (28). This popular account was the first that received
any attention from the Computer Science community.

1978

With M. Rem, proposed the foundations of a Complexity Theory for VLSI, in
which time, area, and energy were the dimensions of a cost vector (29, 30). This work has
led to a fundamental expansion in the notion of computational complexity.

1979

Gave the first public discussion of the role of silicon foundries in promoting
technological innovation (31). The first Caltech conference on VLSI, at which that talk
was given, was also the occasion of the first description of Dave Johannsen’s graduate
research on silicon compilation, and was the first time the term had been coined. The
coincidence of silicon compilation (now called Synthesis) and Silicon Foundries (32) led
to an entire new business model for the semiconductor industry, now called Fabless
Semiconductor. This segment is currently responsible for over half the economic value of
the entire semiconductor industry.

Articulated the impact that the VLSI technology would have on Computer Science
education, a set of predictions that have now appeared in the standard curricula (33).

The book, Introduction to VLSI Systems, written with Lynn Conway as co-author,
appeared (34). This book captured many of the insights of the previous 10 years’ work in
a form that could be taught to students with a wide variety of backgrounds.

1982

With M. Chen, presented the first formal semantics for general VLSI systems (35).
This work led to a completely general hierarchical approach to system specification and
simulation (36, 37, 38, 39).

1983

With T. Lin, extended the hierarchical semantics work to include a physically based
treatment of time delay (40, 41, 42).

1985

With J. Wawrzynek, described a very general concurrent computational approach to
problems requiring the solution of finite-difference equations in time. Used the approach
to produce high-quality musical instruments in real time. The structure used for this
application was a programmable interconnect technology that became the basis for a
large class of commercial Field-Programmable Gate Arrays (43, 44, 45).

1988

With M. A. Mahowald, described the first analog silicon retina (46). The approach
to silicon models of certain neural computations expressed in this chip, and its
successors, foreshadowed a totally new class of physically based computations inspired
by the neural paradigm. More recent results demonstrated that a wide range of visual
and auditory computations of enormous complexity can be carried out in minimal area
and with minute energy dissipation compared with digital implementations.

1989

The book Analog VLSI and Neural Systems was published (47). This book
condensed the insights gained during the previous eight years of work into a single
volume, accessible to students with a wide range of backgrounds. Several recent reviews
have spelled out in some detail the compelling advantages of realizing adaptive systems
directly in analog VLSI. Reduction of system power dissipation by a factor of 10,000,
and of silicon area by a factor of 100 are being demonstrated.

1985-1998

Experience gained in using photo-response of semiconductor structures for
barrier-energy and band-gap studies led to system-level structures that sensed and
processed images in various ways. With numerous collaborators, a large variety of
imaging structures were developed. One branch of this effort resulted in CMOS imagers,
now the most prevalent of all image sensors. A particular subset of these, the X3 sensors,
have produced some of the finest images ever captured by any photographic technology.

1972-2000

Throughout the entire period, worked to bring about a general awareness of
Computation as a physical process, rather than purely a mathematical one. Strongly
advocated the importance of unifying technology and architecture into a single discipline,
and emphasized the importance of this unity for the future of the field at large.

2000

The book Collective Electrodynamics: Quantum Foundations of Electromagnetism,
published by MIT Press, unifies electromagnetic phenomena with the quantum nature of
matter (48).

2007-2011

Recent work on Collective Electrodynamics is evolving an entire
introductory level physics course based on macroscopic quantum systems. This approach
allows students to develop a deep intuition for fundamental physical processes by way of
simple laboratory experiments.

2011-present

G4v - an Engineering approach to Gravitation

G4v is an internally-consistent, quantum coupled treatment of gravitation and electromagnetism. This theory is a direct extension of Einstein's 1911/12 approach. It diff ers from previous attempts in a number of important ways:

The theory is based on Mach's Principle and provides a conceptual
base for the Equivalence Principle.

It is not a metric theory; it is formulated in flat space-time.

Neither the electromagnetic nor gravitational fields are quantized.
The wave functions and four-potentials are continuous functions of
space and time. Quantization results from the interaction of matter and field wave
functions.

The speed of light c is equal to the gravitational potential.
It is not constant, but varies with position and time.

The quantity of matter coupled gravitationally is not the mass m;
it is the Compton wave number.

The theory is based on four-vector coupling.
It is thus locally Lorentz-invariant in regions where the speed of light
can be considered constant.

The source of the electrical four-potential is the charge/current density
four-vector, and that for the gravitational four potential is the
energy-momentum four-vector. Both quantities are de fined for the wave
function of the source matter, and appear as terms in the a ffected
matter wave function.

In addition to obtaining the correct value for the light-deflection problem,
G4v also obtains exactly the same expressions as GR, to the fi rst order
beyond
Newton, for perihelion precession, Gravity Probe B, gravitational redshift
and Shapiro delay. The total gravitational radiation from binary systems
is identical for circular orbits, but has a very slightly di fferent
eccentricity
dependence, which may be testable as more high-eccentricity binary-pulsar
systems are discovered.

The G4v predictions for gravitational-wave radiation patterns from binary
systems, and for the antenna patterns of observatories like LIGO are
markedly di fferent from those of GR. (50, 51)
These predictions should be testable within the next few years.